That's a very Outlook-specific feature. I don't know a mail client that has it. Doing subject change in IMAP is quite expensive because it requires to copy the message and delete the original one, plus some listing updates, etc. So, there's no such feature in Roundcube. You could do this using "Edit as new" feature then save to original message folder, finally delete the original mail, but that's really not a solution, right?
There are (or might be) some plugins that add support for message annotations or tags. I think this would solve the problem as well. Of course, one could write a plugin that does subject change too.
